1. Field of the Invention
The present invention relates to a porous optical fiber preform dehydration and vitrification apparatus for dehydrating and vitrifying a porous optical fiber preform.
More specifically, the present invention relates to a porous optical fiber preform dehydration and vitrification apparatus for improving the air-tightness of an upper portion of a furnace tube for introducing and elevating and lowering the porous optical fiber preform.
Still more specifically, the present invention relates to a porous optical fiber preform dehydration and vitrification apparatus having a structure capable of shortening a dehydration and vitrification time and having a controlling means. Further, the present invention relates to a method of heat treatment of a porous optical fiber preform for adequate control of temperature by using such a dehydration and vitrification apparatus.
Still more specifically, the present invention relates to a porous optical fiber preform dehydration and vitrification apparatus free from damage to the furnace tube and capable of being extended in service life even if used for a large-sized porous optical fiber preform.
Still more specifically, the present invention relates to a porous optical fiber preform dehydration and vitrification apparatus combining all of the above.
2. Description of the Related Art
A single mode optical fiber having for example a diameter of a core of 10 xcexcm and a diameter of a cladding formed on an outer circumference of the core of 125 xcexcm is produced by drawing an optical fiber preform in a heated state.
Such an optical fiber preform is produced by dehydrating and vitrifying a porous optical fiber preform produced by a vapor axial deposition (VAD) method, an outer vapor deposition (OVD) method, or the like.
When dehydrating and vitrifying such a porous optical fiber preform produced by the VAD process, the OVD process. etc., use is made of for example a porous optical fiber preform dehydration and vitrification apparatus illustrated in FIG. 1.
A furnace tube 2 is provided in the dehydration and vitrification apparatus, and a heating furnace 8 is provided around the periphery of the furnace tube 2. By gradually lowering the porous optical fiber preform 1 downward in the furnace tube 2 from above and passing it through a high temperature portion of the heating furnace 8 having a heater 7 accommodated in a furnace body 6 disposed at the outer circumference of the furnace tube 2, the porous optical fiber preform 1 is first dehydrated. At the time of this dehydration, helium (He) gas, a chlorine-containing gas, or the like is passed inside the furnace tube 2. Also, the temperature in the furnace tube 2 is made for example about 1100 to 1300xc2x0 C.
After this dehydration ends, the porous optical fiber preform 1 is pulled up once from the interior of the upper portion of the furnace tube 2 of the dehydration and vitrification apparatus, the temperature in the furnace tube 2 is raised to for example 1500 to 1600xc2x0 C., and the dehydrated porous optical fiber preform 1 is lowered into the furnace tube 2 again from above the furnace tube 2 to heat the porous optical fiber preform 1 by the heater 7 and vitrify it to form an optical fiber preform. At the time of this vitrification, He gas may be passed through the furnace tube 2 or a gas the same as that at the dehydration may be passed.
After the dehydration, the porous optical fiber preform 1 is sometimes doped by adding a dopant for forming a refractive index distribution. This doping is carried out by feeding an atmospheric gas (mainly He gas) and a doping gas (gas containing either of fluorine, boron, or chlorine) into the furnace tube 2 and controlling the temperature in the furnace tube 2 to for example about 1000 to 1300xc2x0 C. (this temperature differs according to the doping gas).
Further, the doping in the dehydration and vitrification apparatus may be carried out simultaneously with the vitrification. In this case, the doping is carried out by incorporating the doping gas into the atmospheric gas in an ordinary vitrification furnace tube 2.
As disclosed in Japanese Unexamined Utility Model Publication (Kokai) No. 6-59438, there has also been proposed a method of dehydration and vitrification by arranging a plurality of heaters (heat sources) in a longitudinal direction of the furnace tube at the outer circumference of the furnace tube in place of movement of the porous optical fiber preform 1 at the time of such heat treatment, sequentially switching the plurality of heaters along the longitudinal direction of the porous optical fiber preform, and therefore sequentially heating the heaters to predetermined temperatures without moving the porous optical fiber preform.
Further, as disclosed in Japanese Unexamined Patent Publication (Kokai) No. 9-110456, there also has been known a method of dehydrating and vitrifying a porous optical fiber preform formed by the VAD process or the OVD process, drawing this, depositing fine glass particles synthesized in a burner flame on the surface using this as a target to prepare a porous optical fiber preform (such a porous optical fiber preform being referred to as a xe2x80x9cporous optical fiber preform with targetxe2x80x9d in the present specification), where when no dopant changing the refractive index distribution in advance is contained in the porous layer, the dehydration and vitrification are performed by heating the porous optical fiber preform with target all at once by a plurality of heat sources.
According to the two above heat treatment methods using the plurality of heaters, since it is not necessary to lower the porous optical fiber preform, there is the advantage that the treatment time can be shortened compared with the method of heat treatment by a dehydration and vitrification apparatus using one heater.
However, the method of heat treatment using a plurality of heaters suffers from the problem that it is hard to apply this method to dehydration and vitrification of a porous optical fiber preform containing a dopant forming a refractive index distribution in advance (this will be referred to as a xe2x80x9cporous optical fiber preform without targetxe2x80x9d in the present specification). The reason for this will be explained. In dehydration, the dopant contained in the porous optical fiber preform in advance reacts, so it is difficult to make the concentration of chlorine gas uniform in the longitudinal direction of the porous optical fiber preform and, as a result, it is difficult to make the refractive index distribution of the porous optical fiber preform uniform in the longitudinal direction of the porous optical fiber preform. Further, it is also difficult to make the temperature in the furnace uniform in the longitudinal direction. Therefore, even if the concentration of chlorine gas could be made uniform in the longitudinal direction, it would be difficult to make the refractive index distribution of the porous optical fiber preform uniform in the longitudinal direction of the porous optical fiber preform.
Even in the case of the xe2x80x9cporous optical fiber preform with targetxe2x80x9d in which a dopant changing the refractive index is not contained in the porous layer at the surface, there was a problem that the dehydration temperature could only be raised to a temperature of the same extent as that for the xe2x80x9cporous optical fiber preform without targetxe2x80x9d, for example about 1300xc2x0 C. at most. This is because due to the treatment at a constant temperature for a long time, for example about 2 to 6 hours, if the entire porous optical fiber preform is raised to a high temperature, firing proceeds from the surface of the porous optical fiber preform and dehydration becomes difficult.
Further, where doping the porous optical fiber preform, there was a problem that the distribution of the dopant of the porous optical fiber preform after the heat treatment did not become uniform in the longitudinal direction. As the cause of this, inadequate temperature hysteresis of the porous optical fiber preform at the time of heat treatment of the porous optical fiber preform can be considered.
A porous optical fiber preform dehydration and vitrification apparatus will be explained in more detail next by referring to FIG. 1.
FIG. 1 is a vertical sectional view of the general configuration of a porous optical fiber preform dehydration and vitrification apparatus according to the related art.
The porous optical fiber preform dehydration and vitrification apparatus illustrated in FIG. 1 has a furnace tube 2 made of quartz glass, carbon, or a ceramic such as alumina for accommodating a porous optical fiber preform 1 to be heat treated for dehydration and vitrification. The dehydration and vitrification apparatus further has an upper lid 31 made of quartz glass, carbon, or a ceramic such as alumina detachably attached to an upper flange 2b of the furnace tube 2 so as to shut an upper opening 2a of furnace tube 2 for inserting and pulling out the porous optical fiber preform 1, an elevating shaft 41 made of quartz glass or a ceramic such as alumina penetrating through this upper lid 31 so that it can freely elevate, and a preform holder 5 provided at the bottom end of this elevating shaft 41 and holding a starting rod 1a at the upper portion of the porous optical fiber preform 1. The dehydration and vitrification apparatus further has a heating furnace 8 provided with a furnace body 6 provided around the outer circumference of the furnace tube 2 and a heater 7 provided in this furnace body 6 for heating the porous optical fiber preform 1 in the furnace tube 2 by the heater 7. The dehydration and vitrification apparatus has a gas feed port 9 for feeding an internal gas into the furnace tube 2 from a lower portion of the furnace tube 2, a gas discharge port 10 for discharging the gas in the furnace tube 2 from the upper side of the furnace tube 2, a gas feed port 11 for feeding an inert gas into the furnace body 6, and an annular furnace tube upper sealing gas feeder 12 which is provided interposed between the upper flange 2b of furnace tube 2 and the upper lid 31 to seal the upper opening 2a of the furnace tube 2.
The elevating shaft 41 is elevated and lowered by a not illustrated elevating mechanism arranged above the furnace tube 2 and is rotated around the center of its axis by a not illustrated rotation mechanism such as a motor. Further, exhaust gas in the furnace tube 2 is discharged from the gas discharge port 10 of the furnace tube 2, passes through an exhaust pipe 13 and a pressure control valve 14 provided at the middle thereof, and is fed to a not illustrated discharge gas treatment device.
The exhaust from the interior of the furnace body 6 passes through an exhaust pipe passage 15 and a pressure control valve 16 provided at the middle thereof and is fed into the not illustrated discharge gas treatment device. The pressure in the furnace 8 and the differential pressure between the gas pressure in the furnace body 6 and the gas pressure in the furnace tube 2 are detected at a pressure meter 17a and a differential pressure meter 17b. The pressure control valves 14 and 16 are controlled by a not illustrated controller so that the differential pressure becomes constant.
The reason for using a furnace tube 2 made of quartz glass, carbon, or a ceramic such as alumina in the porous optical fiber preform dehydration and vitrification apparatus illustrated in FIG. 1 is that a halogen-based gas is used at the time of heat treatment of the porous optical fiber preform 1, so this gas must be prevented from diffusing into the ambient atmosphere or entering into the furnace body 6.
In the porous optical fiber preform dehydration and vitrification apparatus illustrated in FIG. 1, the upper lid 31 is detached from the furnace tube 2, the porous optical fiber preform 1 is inserted into the furnace tube 2 by a lowering operation of the not illustrated elevating mechanism, the upper lid 31 is placed over the upper opening 2a of the furnace tube 2 so that atmospheric air (outside air) does not enter into this furnace tube 2, and the porous optical fiber preform 1 is dehydrated or vitrified in the atmosphere of the gas fed from the gas feed port 9 into the furnace tube 2 while lowering the porous optical fiber preform 1 in the furnace tube 2 by the lowering operation of the not illustrated elevating mechanism and while rotating the porous optical fiber preform 1 around the center of its axis by the rotation operation of the not illustrated rotation mechanism.
At the time of this heat treatment, the heat porous optical fiber preform 1 is heat treated while rotating it around the center of its axis as explained above, therefore, with an elevating shaft 41 and an upper lid 31 made of quartz glass or a ceramic such as alumina, a certain degree of clearance 18 must be provided between the elevating shaft 41 and the upper lid 31 in view of the level of machining precision of the materials. Accordingly, gas sealing has been carried out by an inert gas such as nitrogen gas or argon gas ejected from the furnace tube upper sealing gas feeder 12 so that the atmospheric air (outside air) does not enter into the furnace tube 2 through this clearance 18.
In the sealing structure of the elevating shaft passage of the upper lid 31 through which the elevating shaft 41 shown in FIG. 1 penetrates, however, a sufficient sealing performance cannot be obtained at the time of heat treatment by bringing the interior of the furnace tube 2 into a depressurized state (or a vacuum state) or a pressurized state.
In order to raise the air-tightness between the elevating shaft 41 and the upper lid 31, as disclosed in for example Japanese Unexamined Patent Publication (Kokai) No. 62-27343, it has been proposed to perform the sealing by providing a seal member made of an O-ring in the elevating shaft passage of the upper lid 31 through which the elevating shaft 41 passes. In the sealing structure disclosed in Japanese Unexamined Patent Publication (Kokai) No. 62-27343, however, only an O-ring is provided as the seal member, so there is the inconvenience that the O-ring is thermally damaged due to the heat at the time of heat treatment. The durability of the seal member is low and this structure is hard to put into practical use.
Further, in order to raise the air-tightness between the elevating shaft 41 and the upper lid 31, as disclosed in for example Japanese Unexamined Patent Publication (Kokai) No. 4-18626, it has been proposed to perform the sealing by providing a seal member made of carbon fiber in the elevating shaft passage of the upper lid 31 through which the elevating shaft 41 passes. In the sealing structure disclosed in Japanese Unexamined Utility Model Publication (Kokai) No. 4-18626, however, there is the inconvenience that the carbon fiber used as the seal member generates dust due to abrasion of the carbon fiber at the elevation of the elevating shaft 41, this dust enters into the furnace tube 2 and therefore a foreign substance adheres to the porous optical fiber preform 1.
Further, in the porous optical fiber preform dehydration and vitrification apparatus illustrated in FIG. 1, when the amount of the gas (mainly the He gas) fed into the furnace tube 2 is reduced, the optical fiber transmission characteristic tends to become degraded. For this reason, the He gas must be sufficiently fed, but He gas is expensive, so production of the porous optical fiber preform becomes higher in cost. Conversely speaking, the amount of the expensive He gas fed cannot be reduced in the illustrated porous optical fiber preform dehydration and vitrification apparatus.
When the outside diameter of the porous optical fiber preform becomes larger and the diameter of the furnace tube 2 becomes larger as in recent years, the amount of the He gas fed is increased, so the problem of the amount of feed of the He gas becomes larger.
If forming the furnace tube 2 by a quartz material, when the heating temperature by the heater 7 becomes 1300 to 1400xc2x0 C. or more, the furnace tube 2 becomes soft and deforms. In order to prevent this deformation, there was the restriction that the pressure in the furnace tube 2 had to be made higher than the pressure in the furnace body 6 by several mmAq to several tens of mmAq.
Further, if a carbon heater is used as the heater 7, the pressure in the furnace body 6 must be made higher than the atmospheric pressure by several mmAq, therefore ordinarily the pressure of the furnace tube 2 made higher than the atmospheric pressure by ten or so mmAq. If feeding an inert gas such as nitrogen gas or argon gas from the furnace tube upper sealing gas feeder 12 for the sealing so that the atmosphere does not enter through the clearance 18 between the upper lid 31 and the elevating shaft 41, Due to the gas in the furnace tube 2 higher than the atmospheric pressure by ten and several mmAq, there was the problem that a considerably large amount of sealing gas became necessary.
Further, along with the increase in demand for optical fiber in recent years, the porous optical fiber preforms 1 for optical fibers have become larger, that is, they have become larger in diameter and longer in length. In order to heat such a large porous optical fiber preform to and dehydrate and vitrify (sinter) it, a large-sized heating furnace becomes necessary. However, when dehydrating and firing a large porous optical fiber preform, since a furnace tube 2 made of quartz would be heated to 1500xc2x0 C. or more over a wide range, there is a possibility that the furnace tube 2 would become soft and the furnace tube 2 would buckle and deform due to its own weight.
An object of a first aspect of the present invention is to provide a porous optical fiber preform dehydration and vitrific ation apparatus capable of improving the sealing performance between the upper lid and the elevating shaft or between the upper lid and the furnace tube or the furnace body.
Another object of the first aspect of the present invention is to provide a porous optical fiber preform dehydration and vitrification apparatus resistant to thermal damage even if the upper lid is formed by a metal and a seal member made of rubber or a resin is interposed between the upper lid and the elevating shaft.
Still another object of the first aspect of the present invention is to provide a porous optical fiber preform dehydration and vitrification apparatus capable of easily performing heat treatment by bringing the interior of the furnace tube into a depressurized state (or a vacuum state) or a pressurized state.
Still another object of the first aspect of the present invention is to provide a porous optical fiber preform dehydration and vitrification apparatus resistant to corrosion by the treatment gas even if the upper lid and the elevating shaft are made of a metal.
Still another object of the first aspect of the present invention is to provide a porous optical fiber preform dehydration and vitrification apparatus capable of preventing radiant heat in the furnace tube from being conducted to the upper lid.
Still another object of the first aspect of the present invention is to provide a porous optical fiber preform dehydration and vitrification apparatus capable of suppressing the flow of the sealing gas into a treatment chamber containing the porous optical fiber preform.
Still another object of the first aspect of the present invention is to provide a porous optical fiber preform dehydration and vitrification apparatus capable of performing heat treatment of the porous optical fiber preform without moving the porous optical fiber preform in the furnace tube.
An object of a second aspect of the present invention is to provide a method of heat treatment of a porous optical fiber capable of shortening the treatment time of the heat treatment required in the porous optical fiber preform dehydration and vitrification apparatus.
Another object of the second aspect of the present invention is to provide a method of heat treatment of a porous optical fiber, of a type performing the required heat treatment in a state where the elevation or descent of the porous optical fiber preform to be treated in the porous optical fiber preform dehydration and vitrification apparatus is suspended, which can shorten the heat treatment time.
Still another object of the second aspect of the present invention is to provide a method of heat treatment of a porous optical fiber, of a type performing the required heat treatment while moving the porous optical fiber preform to be treated in the porous optical fiber preform dehydration and vitrification apparatus, which can shorten the time of the required heat treatment.
An object of a third aspect of the present invention is to provide a porous optical fiber preform dehydration and vitrification apparatus capable of preventing buckling and deformation of the furnace tube at the time of high temperature heating due to its own weight.
Another object of the third aspect of the present invention is to provide a porous optical fiber preform dehydration and vitrification apparatus capable of preventing buckling and deformation of the furnace tube at the time of high temperature heating due to its own weight by utilizing muffle pipes.
An object of a fourth aspect of the present invention is to provide a porous optical fiber preform dehydration and vitrification apparatus which suitably combines the features of the porous optical fiber preform dehydration and vitrification apparatuses of the first to third aspects and can achieve all of the above objects.
First, the porous optical fiber preform dehydration and vitrification apparatus of the first aspect of the present invention will be explained.
The porous optical fiber preform dehydration and vitrification apparatus of the first aspect of the present invention is provided with a furnace tube made of quartz glass, a ceramic, etc, for accommodating a porous optical fiber preform to be treated, an upper lid detachably attached to an upper portion of the furnace tube so as to shut an upper opening of the furnace tube for inserting and pulling up the porous optical fiber preform, an elevating shaft penetrating through the upper lid so that it can freely elevate, a preform holder provided at the bottom end of the elevating shaft and holding the upper portion of the porous optical fiber preform, a heating furnace provided around the outer circumference of the furnace tube and heating the porous optical fiber preform in the furnace tube by a heater, a gas feed port for feeding the gas from the lower portion to the internal portion of the furnace tube, and a gas discharge port for discharging the gas in the furnace tube at the upper side of the furnace tube. This porous optical fiber preform dehydration and vitrification apparatus is characterized in that the upper lid is formed by a metal, and the preform holder is formed by quartz glass or a ceramic.
A corrosion-resistant layer can be provided on at least an inner surface of the upper lid.
A seal member made of rubber or a resin can be provided at the elevating shaft passage of the upper lid through which the elevating shaft penetrates so that the elevating shaft can elevate while the sealed state is held.
The sealing can be carried out between the upper lid and the furnace tube or furnace body by the seal member made of rubber or a resin.
A cooling means for cooling the upper lid by a cooling medium can be provided at the upper lid.
By forming the upper lid by a metal in this way, the machining of the seal member for forming the sealing portion between the upper lid and the elevating shaft becomes easy, the clearance between the upper lid and the elevating shaft can be made as small as possible, and the two can be easily sealed by the seal member made of rubber or a resin.
If sealing between the upper lid and the elevating shaft and between the upper lid and the furnace tube or the furnace body by the seal member made of rubber or a resin, reliable sealing is possible without generating dust in the upper portion of the furnace tube.
If reliable sealing is possible between the upper lid and the elevating shaft and between the upper lid and the furnace tube or furnace body, the heat treatment by bringing the interior of the furnace tube into the vacuum state or the pressurized state becomes easy.
Even if the seal member is made of rubber or a resin, the upper lid is cooled by the cooling means by the cooling medium, so the thermal damage of the seal member can be further effectively prevented.
Further, when both of the upper lid and the elevating shaft are formed by a metal, the precision of the sealing part can be further improved and reliable sealing becomes possible. In this case, desirably the corrosion-reslstant layer is provided at the surface of the elevating shaft.
Even when the elevating shaft is made of a metal, since the preform holder is formed by quartz glass or a ceramic, an intrusion of a foreign substance from the preform holder near the porous optical fiber preform into the porous optical fiber preform can be avoided as much as possible.
Even if both of the upper lid and the elevating shaft are made of a metal, since the corrosion-resistant layer is provided on at least the inner surface of the upper lid and the surface of the elevating shaft, corrosion of them by the treatment gas can be prevented.
In this case, preferably a heat insulating material is provided at the upper lid covering the inner surface thereof. When providing this, the conductance of the radiant heat from the heater to the upper lid can be reduced, and the thermal damage of the seal member can be prevented.
Preferably, the upper lid is provided with an inert gas passage for passing an inert gas covering the inner surface thereof and covering the surface of the elevating shaft projecting into the upper lid. When providing this, the treatment gas can be kept from reaching the metal upper lid and elevating shaft by the inert gas flowing through this inert gas passage.
Preferably, a heat insulating means for preventing the radiant heat in the furnace tube from being conducted to the upper lid is supported at the upper portion of the preform holder. By doing this, the temperature rise of the upper lid due to the radiant heat in the furnace tube can be suppressed, and the thermal damage of the seal member made of rubber or resin can be more effectively prevented.
Preferably, a gas blocking means for keeping the sealing gas sealed between the upper lid and the elevating shaft penetrating through this from flowing into the treatment chamber containing the porous optical fiber preform is provided between the upper lid and the heat insulating means. By doing this, the flow of the sealing gas into the treatment chamber containing the porous optical fiber preform can be suppressed. For this reason, degradation of the transmission characteristic of the optical fiber due to the reduction of the treatment gas (mainly He gas) to be fed into the treatment chamber can be prevented. In this way, according to the present invention, the amount of the expensive He gas used can be reduced. Even when the diameter of the furnace tube becomes large, the degradation of the transmission characteristic of the optical fiber can be suppressed.
The heat insulating means is preferably always arranged lower than the gas discharge port during the period when the porous optical fiber preform is treated. By doing this, the sealing gas entering into the furnace tube is discharged to the outside from the gas discharge port of the furnace tube, and the flow of the sealing gas into the treatment chamber containing the porous optical fiber preform can be further effectively reduced.
The gas blocking means is preferably always arranged higher than the gas discharge port during the period when the porous optical fiber preform is heat treated. By doing this, the part of the furnace tube between the upper lid and the gas blocking means acts as a buffer chamber. The gas pressure in this buffer chamber can be made a little higher than the part connected to the gas discharge port, so the amount of the sealing gas can be reduced.
In the upper portion of the preform holder, preferably the gas blocking and insulating means which prevents the conductance of the radiant heat in the furnace tube to the upper lid and always arranged lower than the gas discharge port during the period when the porous optical fiber preform is heat treated is supported. By doing this, the increase of the radiant heat from the heater and conductance of it to the upper lid can be reduced and therefore the thermal damage of the seal member can be prevented. In addition, the flow of the sealing gas into the treatment chamber containing the porous optical fiber preform can be suppressed, therefore the degradation of the transmission characteristic of the optical fiber due to the reduction of the treatment gas (mainly He gas) to be fed into the treatment chamber can be suppressed and thus the amount of the expensive He gas used can be reduced. Further, even when the diameter of the furnace tube becomes large, the degradation of the transmission characteristic of the optical fiber can be suppressed.
Preferably, heaters are provided in a plurality of stages in a direction toward the longitudinal direction of the porous optical fiber preform. By doing this, by controlling the supply of power to the different heaters, an intended position of the quartz porous optical fiber preform in the longitudinal direction can be heated without moving the porous optical fiber preform in the furnace tube, therefore there are the advantages that it becomes possible to reduce the rubbing at a shaft sealed part along with the elevation or lowering of the elevating shaft, achieve an extension of the service life, and eliminate the strict requirement on the machining precision of the elevating shaft.
Next, a porous optical fiber preform dehydration and vitrification apparatus according to the second aspect of the present invention will be explained.
The second aspect of the present invention improves the method of heat treatment of a porous optical fiber preform which arranges a plurality of heat sources along the longitudinal direction around the outer circumference of a furnace tube in a porous optical fiber preform dehydration and vitrification apparatus, arranges the porous optical fiber preform in its heating furnace, and thereby performs the required heat treatment on the porous optical fiber preform.
The present inventors engaged in various studies and as a result discovered that dehydration, doping, vitrification, and other heat treatment occur through two time periods: (a) a time of temperature rise required for raising the temperature of the porous optical fiber preform up to the treatment temperature and (b) a time where the porous optical fiber preform reaches the required heat treatment temperature and the reactions of the dehydration, doping, and vitrification sufficiently advance.
Further, the present inventors discovered that the higher the dehydration temperature, the faster the reaction, but conversely sufficient dehydration was no longer possible since the surface of the porous optical fiber preform began to be fired and therefore the surface of the porous optical fiber preform became densified.
The present inventors discovered from the above that, in order to shorten the treatment time of the dehydration or the vitrification, (1) it was important to preheat the porous optical fiber preform and (2) if the treatment time was short, the heat treatment temperature of the porous optical fiber preform was preferably made high.
Further, the present inventors discovered that, in order to efficiently dope the preform, the density of at least the surface part of the porous part of the porous optical fiber preform was desirably made high immediately after the doping.
Therefore, in the method of heat treatment of porous optical fiber preform of the second aspect of the present invention, before the required heat treatment on the porous optical fiber preform, the entire porous optical fiber preform is preheated up to a predetermined preheating temperature lower than the required heat treatment temperature. In this state, the porous optical fiber preform is heat treated as required while moving the position where the porous optical fiber preform becomes the required heat treatment temperature from the preheating temperature in the longitudinal direction of the porous optical fiber preform.
If preheating the entire porous optical fiber preform up to a predetermined preheating temperature lower than the required heat treatment temperature in advance before performing the required heat treatment on the porous optical fiber preform in this way, it is possible to shorten the time for raising the temperature of the porous optical fiber preform up to the temperature of the required heat treatment and perform the required heat treatment over the entire length of the porous optical fiber preform. Further, when performing the required heat treatment on the porous optical fiber preform while moving the position where the porous optical fiber preform becomes the required heat treatment temperature in the longitudinal direction of the porous optical fiber preform, it is possible to perform the required heat treatment at an almost uniform temperature over the entire length of the porous optical fiber preform and to make the refractive index distribution of the porous optical fiber preform almost completely uniform in the longitudinal direction of the porous optical fiber preform.
In the second aspect of the present invention, preferably the heating is carried out so that the rates of temperature rise from the preheating temperature until reaching the required heat treatment temperature become substantially constant at all portions in the longitudinal direction of the porous optical fiber preform. Further, more preferably the heating is carried out so that the rates of temperature fall from the required heat treatment temperature to the preheating temperature become substantially constant at all portions in the longitudinal direction of the porous optical fiber preform.
By doing this, all parts of the porous optical fiber preform in the longitudinal direction can be raised in temperature at a constant rate of temperature rise for heat treatment and the quality of the parts of the porous optical fiber preform in the longitudinal direction after the heat treatment can be stabilized. In this case, if the rate of temperature fall for lowering the parts of the porous optical fiber preform in the longitudinal direction from the required heat treatment temperature to the preheating temperature becomes substantially constant, the quality of the parts of the porous optical fiber preform in the longitudinal direction after heat treatment can be stabilized much better.
Further, in the second aspect of the present invention, as one of operations of movement of the position where the porous optical fiber preform becomes the required heat treatment temperature in the longitudinal direction of the porous optical fiber preform, the temperatures of a plurality of heat sources (heaters) arranged in the longitudinal direction around the outer circumference of the furnace tube are sequentially controlled along the longitudinal direction of the furnace tube so that the temperatures of the parts of the furnace tube corresponding to the heat sources become the required heat treatment temperature from preheating temperatures lower than the required heat treatment temperature.
When employing such a method, the operation of moving the position where the porous optical fiber preform becomes the required heat treatment temperature in the longitudinal direction of the porous optical fiber preform can be easily carried out by just controlling the conditions of supply of power to the plurality of heat sources etc.
Further, in the second aspect of the present invention, when using a plurality of heat sources to heat the porous optical fiber preform, preferably the first position at which the porous optical fiber preform is raised from the preheating temperature to the required heat treatment temperature is made the substantial center of the porous optical fiber preform in the longitudinal direction and the position where the porous optical fiber preform becomes the required heat treatment temperature from the preheating temperature is moved from that position toward the two ends of the porous optical fiber preform in the longitudinal direction for the required heat treatment on the porous optical fiber preform.
When employing such a heat treatment method, the heat treatment time with respect to the entire length of the porous optical fiber preform can be shortened to about a half compared with that by a method of heat treatment from one end of the porous optical fiber preform to the other end.
Further, in the second aspect of the present invention, as another operation moving the position where the porous optical fiber preform becomes the required heat treatment temperature in the longitudinal direction of the porous optical fiber preform, the temperature of a specific part of the heat sources arranged along the longitudinal direction around the outer circumference of the furnace tube is set so that the temperature in the furnace tube at the corresponding position becomes the required heat treatment temperature, the temperatures of the remaining parts of the heat sources are set so that the temperatures in the furnace tube at the corresponding positions become a preheating temperature lower than the required heat treatment temperature, and the heat treatment is carried out while moving the porous optical fiber preform in the longitudinal direction so that portions of the porous optical fiber preform sequentially face the specific part of the heat sources.
When employing such a method, since the operation of moving the position where the porous optical fiber preform becomes the required heat treatment temperature in the longitudinal direction of the porous optical fiber preform can be carried out by setting the temperature of a specific part of the heat sources arranged along the longitudinal direction around the outer circumference of the furnace tube so that the temperature in the furnace tube at the corresponding position becomes the required heat treatment temperature and setting the temperatures of the other parts of the heat sources so that the temperatures in the furnace tube at the corresponding portions become the preheating temperatures or post-treatment temperatures lower than the required heat treatment temperature, there are the advantages that the part of the heat sources to be set so that the furnace tube becomes the required heat treatment temperature becomes small and temperature control of the heat sources to be set so that the furnace tube becomes the required heat treatment temperature becomes easy.
Further, in the second aspect of the present invention, where the heat treatment of the porous optical fiber preform is doping, by carrying this out between the dehydration and the vitrification and making the temperature for uniformly heating the entire porous optical fiber preform a temperature by which densification of the surface part of the porous optical fiber preform starts, it is possible to densify the entire surface of the porous optical fiber preform in a short time and to suppress dispersal of the dopant from the surface of the porous optical fiber preform. Note that when adding the doping gas into the atmosphere of the preheating before the vitrification to be performed after the end of the doping, the dispersal of the dopant from the surface of the porous optical fiber preform can be further suppressed.
Further, in the second aspect of the present invention, it is also possible to perform the doping at the time of the vitrification. In this case, the porous optical fiber preform is sufficiently preheated by the preheating, so the temperature difference in the diametrical direction of the porous optical fiber preform can be reduced. Further, the rate of change of the temperature in the longitudinal direction is made substantially constant at the time of the vitrification, so the temperature difference in the longitudinal direction of the porous optical fiber preform can be reduced. Accordingly, the doping can be carried out substantially uniformly with respect to the entire porous optical fiber preform.
Further, in the second aspect of the present invention, the heat treatment of the porous optical fiber preform may be any of dehydration, doping, or vitrification of the porous optical fiber preform.
The porous optical fiber preform dehydration and vitrification apparatus of the third aspect of the present invention will be explained next.
According to the third aspect of the present invention, there is provided a porous optical fiber preform dehydration and vitrification apparatus provided with a furnace tube which penetrates through the center of a furnace body for accommodating a porous glass preform and a heating portion provided with a heating element arranged around the periphery of the furnace tube in the furnace body for heating the porous optical fiber preform in the furnace tube, wherein a furnace tube weight distributing means for distributing the weight of the furnace tube in its longitudinal direction is provided around the outer circumference of the furnace tube.
The furnace tube weight distributing means can be provided around the outer circumference of the furnace tube with a plurality of flanges provided at predetermined intervals in the longitudinal direction and a furnace tube weight receiving means for supporting the flanges around the outer circumference of the furnace tube.
The furnace tube weight receiving means can be constituted by muffle pipes which is interposed between vertically adjoining flanges around the outer circumference of the furnace tube and bears the weight of the furnace tube ac ting upon the upper flanges.
The furnace tube weight receiving means can also be constituted by muffle pipes arranged along the outer circumference of the furnace tube and a plurality of supports which are provided in the muffle pipes corresponding to the flange of the furnace tube and support the flange.
The furnace tube weight receiving means comprises a first muffle pipe provided around the outer circumference of the furnace tube and between vertically adjoining flanges, and bears the weight of the furnace tube acting upon the upper flanges, and a second muffle pipe along the outer circumference of the furnace tube.